Some of the major cities along the length of the arc include Portland, Seattle, and Vancouver, and the population in the region exceeds 10,000,000 people. All could be potentially affected by volcanic activity and great subduction-zone earthquakes along the arc. Because the population of the Pacific Northwest is rapidly increasing, the Cascade volcanoes are some of the most dangerous, due to their eruptive history and potential for future eruptions, and because they are underlain by weak, hydrothermally altered volcanic rocks that are susceptible to failure. Consequently, Mount Rainier is one of the Decade Volcanoes identified by the International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) as being worthy of particular study, due to the danger it poses to Seattle and Tacoma. Many large, long-runout landslides originating on Cascade volcanoes have inundated valleys tens of kilometers from their sources, and some of the inundated areas now support large populations.

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The Cascade Arc includes nearly 20 major volcanoes, among a total of over 4,000 separate volcanic vents including numerous stratovolcanoes, shield volcanoes, lava domes, and cinder cones, along with a few isolated examples of rarer volcanic forms such as tuyas. Volcanism in the arc began about 37 million years ago; however, most of the present-day Cascade volcanoes are less than 2,000,000 years old, and the highest peaks are less than 100,000 years old. Twelve volcanoes in the arc are over 10,000 feet (3,000 m) in elevation, and the two highest, Mount Rainier and Mount Shasta, exceed 14,000 feet (4,300 m). By volume, the two largest Cascade volcanoes are the broad shields of Medicine Lake Volcano and Newberry Volcano, which are about 145 cubic miles (600 km3) and 108 cubic miles (450 km3) respectively. Mount Garibaldi and Glacier Peak are the only two Cascade volcanoes that are made exclusively of dacite.

Over the last 37 million years, the Cascade Arc has been erupting a chain of volcanoes along the Pacific Northwest.[2] Several of the volcanoes in the arc are frequently active. The volcanoes of the Cascade Arc share some general characteristics, but each has its own unique geological traits and history. Lassen Peak in California, which last erupted in 1917, is the southernmost historically active volcano in the arc, while Mount Meager in British Columbia, which erupted about 2,350 years ago, is generally considered the northernmost member of the arc. A few isolated volcanic centers northwest of Mount Meager such as the Silverthrone Caldera, which is a circular 20 km (12 mi) wide, deeply dissected calderacomplex, may also be the product of Cascadia subduction because the igneous rocksandesite, basaltic andesite, dacite and rhyolite can also be found at these volcanoes as they are elsewhere along the subduction zone.[3][4] At issue are the current estimates of plate configuration and rate of subduction, but based on the chemistry of these volcanoes, they are also subduction related and therefore part of the Cascade Volcanic Arc.[5][6] The Cascade Volcanic Arc appears to be segmented; the central portion of the arc is the most active and the northern end least active.

Lavas representing the earliest stage in the development of the Cascade Volcanic Arc mostly crop out south of the North Cascades proper, where uplift of the Cascade Range has been less, and a thicker blanket of Cascade Arc volcanic rocks has been preserved. In the North Cascades, geologists have not yet identified with any certainty any volcanic rocks as old as 35 million years, but remnants of the ancient arc’s internal plumbing system persist in the form of plutons, which are the crystallized magma chambers that once fed the early Cascade volcanoes. The greatest mass of exposed Cascade Arc plumbing is the Chilliwack batholith, which makes up much of the northern part of North Cascades National Park and adjacent parts of British Columbia beyond. Individual plutons range in age from about 35 million years old to 2.5 million years old. The older rocks invaded by all this magma were affected by the heat.

Around the plutons of the batholith, the older rocks recrystallized. This contact metamorphism produced a fine mesh of interlocking crystals in the old rocks, generally strengthening them and making them more resistant to erosion. Where the recrystallization was intense, the rocks took on a new appearance dark, dense and hard. Many rugged peaks in the North Cascades owe their prominence to this baking. The rocks holding up many such North Cascade giants, as Mount Shuksan, Mount Redoubt, Mount Challenger, and Mount Hozomeen, are all partly recrystallized by plutons of the nearby and underlying Chilliwack batholith.

The Pemberton Volcanic Belt is an eroded volcanic belt north of the Garibaldi Volcanic Belt, which appears to have formed during the Miocene before fracturing of the northern end of the Juan de Fuca Plate. The Silverthrone Caldera is the only volcano within the belt that appears related to seismic activity since 1975.

Mount Meager is the most unstable volcanic massif in Canada. It has dumped clay and rock several meters deep into the Pemberton Valley at least three times during the past 7,300 years. Recent drilling into the Pemberton Valley bed encountered remnants of a debris flow that had traveled 50 km (31 mi) from the volcano shortly before it last erupted 2,350 years ago. About 1,000,000,000 cubic metres (0.24 cu mi) of rock and sand extended over the width of the valley. Two previous debris flows, about 4,450 and 7,300 years ago, sent debris at least 32 km (20 mi) from the volcano. Recently, the volcano has created smaller landslides about every ten years, including one in 1975 that killed four geologists near Meager Creek. The possibility of Mount Meager covering stable sections of the Pemberton Valley in a debris flow is estimated at about one in 2,400 years. There is no sign of volcanic activity with these events. However scientists warn the volcano could release another massive debris flow over populated areas any time without warning.

In the past, Mount Rainier has had large debris avalanches, and has also produced enormous lahars due to the large amount of glacial ice present. Its lahars have reached all the way to the Puget Sound. Around 5,000 years ago, a large chunk of the volcano slid away and that debris avalanche helped to produce the massive Osceola Mudflow, which went all the way to the site of present-day Tacoma and south Seattle. This massive avalanche of rock and ice took out the top 1,600 feet (490 m) of Rainier, bringing its height down to around 14,100 feet (4,300 m). About 530 to 550 years ago, the Electron Mudflow occurred, although this was not as large-scale as the Osceola Mudflow.

Indigenous peoples have inhabited the area for thousands of years and developed their own myths and legends concerning the Cascade volcanoes. According to some of these tales, Mounts Baker, Jefferson, Shasta and Garibaldi were used as refuge from a great flood. Other stories, such as the Bridge of the Gods tale, had various High Cascades such as Hood and Adams, act as god-like chiefs who made war by throwing fire and stone at each other. St. Helens with its pre-1980 graceful appearance, was regaled as a beautiful maiden for whom Hood and Adams feuded. Among the many stories concerning Mount Baker, one tells that the volcano was formerly married to Mount Rainier and lived in that vicinity. Then, because of a marital dispute, she picked herself up and marched north to her present position. Native tribes also developed their own names for the High Cascades and many of the smaller peaks, the most well known to non-natives being Tahoma, the Lushootseed name for Mount Rainier. Mount Cayley and The Black Tusk are known to the Squamish people who live nearby as "the Landing Place of the Thunderbird".

Hot springs in the Canadian side of the arc, were originally used and revered by First Nations people. The springs located on Meager Creek are called Teiq[7] in the language of the Lillooet River and were the farthest up the Lillooet River. The spirit-beings/wizards known as "the Transformers" reached them during their journey into the Lillooet Country, and were a "training" place for young First Nations men to acquire power and knowledge. In this area, also, was found the blackstone chief's head pipe that is famous of Lillooet artifacts; found buried in volcanic ash, one supposes from the 2350 BP eruption of Mount Meager.

Legends associated with the great volcanoes are many, as well as with other peaks and geographical features of the arc, including its many hot springs and waterfalls and rock towers and other formations. Stories of Tahoma - today Mount Rainier and the namesake of Tacoma, Washington - allude to great, hidden grottos with sleeping giants, apparitions and other marvels in the volcanoes of Washington, and Mount Shasta in California has long been well known for its associations with everything from Lemurians to aliens to elves and, as everywhere in the arc, Sasquatch or Bigfoot.

With the exception of the 1915 eruption of remote Lassen Peak in Northern California, the arc was quiet for more than a century. Then, on May 18, 1980, the dramatic eruption of little-known Mount St. Helens shattered the quiet and brought the world's attention to the arc. Geologists were also concerned that the St. Helens eruption was a sign that long-dormant Cascade volcanoes might become active once more, as in the period from 1800 to 1857 when a total of eight erupted. None have erupted since St. Helens, but precautions are being taken nevertheless, such as the Mount Rainier Volcano Lahar Warning System in Pierce County, Washington.[8]

Because of the very large fault area, the Cascadia subduction zone can produce very large earthquakes, magnitude 9.0 or greater, if rupture occurred over its whole area. When the "locked" zone stores up energy for an earthquake, the "transition" zone, although somewhat plastic, can rupture. Thermal and deformation studies indicate that the locked zone is fully locked for 60 km (37 mi) downdip from the deformation front. Further downdip, there is a transition from fully locked to aseismic sliding.

The 1980 eruption of Mount St. Helens was one of the most closely studied volcanic eruptions in the arc and one of the best studied ever. It was a Plinian style eruption with a VEI=5 and was the most significant to occur in the lower 48 U.S. states in recorded history. An earthquake at 8:32 a.m. on May 18, 1980, caused the entire weakened north face to slide away. An ash column rose high into the atmosphere and deposited ash in 11 U.S. states. The eruption killed 57 people and thousands of animals and caused more than a billion U.S. dollars in damage.

On May 22, 1915, an explosive eruption at Lassen Peak devastated nearby areas and rained volcanic ash as far away as 200 miles (320 km) to the east.[10] A huge column of volcanic ash and gas rose more than 30,000 feet (9,100 m) into the air and was visible from as far away as Eureka, California, 150 miles (240 km) to the west. A pyroclastic flow swept down the side of the volcano, devastating a 3-square-mile (7.8 km2) area. This explosion was the most powerful in a 1914–17 series of eruptions at Lassen Peak.[10]

The 7,700 BP eruption of Mount Mazama was a large catastrophic eruption in the U.S. state of Oregon. It began with a large eruption column with pumice and ash that erupted from a single vent. The eruption was so great that most of Mount Mazama collapsed to form a caldera and subsequent smaller eruptions occurred as water began to fill in the caldera to form Crater Lake. Volcanic ash from the eruption was carried across most of the Pacific Northwest as well as parts of southern Canada.

Mount Cayley in the Pacific Range last erupted about 20,000 years ago. It contains several complex features which probably represent multiple eruptions under different conditions and are difficult to classify.

During the mid-19th century, Mount Baker erupted for the first time in several thousand years. Fumarole activity remains in Sherman Crater, just south of the volcano's summit, became more intense in 1975 and is still energetic. However, an eruption is not expected in the near future.[13]

Glacier Peak

Glacier Peak last erupted about 200–300 years ago and has erupted about six times in the past 4,000 years.[13]

Mount Rainier

Mount Rainier last erupted between 1824 and 1854, but many eyewitnesses reported eruptive activity in 1858, 1870, 1879, 1882 and in 1894 as well. Mount Rainier has created at least four eruptions and many lahars in the past 4,000 years.[13]

Mount Adams

Mount Adams was last active about 1,000 years ago and has created few eruptions during the past several thousand years, resulting in several major lava flows, the most notable being the A. G. Aiken Lava Bed, the Muddy Fork Lava Flows, and the Takh Takh Lava Flow. One of the most recent flows issued from South Butte created the 4.5-mile (7.2 km) long by 0.5-mile (0.80 km) wide A.G. Aiken Lava Bed. Thermal anomalies (hot spots) and gas emissions (including hydrogen sulfide) have occurred especially on the summit plateau since the Great Slide of 1921.[13]

Mount Hood

Mount Hood was last active about 200 years ago, creating pyroclastic flows, lahars, and a well-known lava dome close to its peak called Crater Rock. Between 1856 and 1865, a sequence of steam explosions took place at Mount Hood.[13]

Mount Shasta last erupted in 1786 and has been the most active volcano in California for about 4,000 years, erupting once every 300 years.[13] The 1786 eruption created a pyroclastic flow, a lahar and three cold lahars, which streamed 7.5 miles (12.1 km) down Shasta's east flank via Ash Creek. A separate hot lahar went 12 miles (19 km) down Mud Creek.

Eleven of the 13 volcanoes in the Cascade Range have erupted at least once in the past 4,000 years, and seven have done so in just the past 200 years.[13] The Cascade volcanoes have had more than 100 eruptions over the past few thousand years, many of them explosive eruptions.[13] However, certain Cascade volcanoes can be dormant for hundreds or thousands of years between eruptions, and therefore the great risk caused by volcanic activity in the regions is not always readily apparent.

Renewed volcanic activity in the Cascade Arc, such as the 1980 eruption of Mount St. Helens, has offered a great deal of evidence about the structure of the Cascade Arc. One effect of the 1980 eruption was a greater knowledge of the influence of landslides and volcanic development in the evolution of volcanic terrain. A vast piece on the north side of Mount St. Helens dropped and formed a jumbled landslide environment several kilometers away from the volcano. Pyroclastic flows and lahars moved across the countryside. Parallel episodes have also happened at Mount Shasta and other Cascade volcanoes in prehistoric times.

Washington has a majority of the very highest volcanoes, with 4 of the top 6 overall, although Oregon does hold a majority of the next highest peaks. Even though Mount Rainier is the tallest, it is not the largest by volume. Mount Shasta in California is the largest by volume, followed by Washington's Mount Adams. Mount Rainier is thus the 3rd largest by eruptive volume. Below is a list of the highest Cascade Volcanoes: